2003 Berlin: 2:04:55
2014 Berlin: 2:02:57
Simple arithmetic could let you to believe 1:59:59 will happen in 15 years or so.
But then, the last decade has seen a sea change in the marathon talent pool, with runners "moving to" the distance from an early age, even their teen years. (Causing the inverse sea change in the now-stagnant 10,000.) We probably won't see a minute coming off the WR every five years once we settle into this "new normal."
But we WILL see sub-2. Before 2040.